1. Field
Exemplary embodiments of the present disclosure relate to optical modulators, and more particularly, to optical modulators having a wide bandwidth based on Fabry-Perot resonant reflection.
2. Description of the Related Art
An image captured by a general camera does not include distance information. In order to realize a 3-dimensional (3D) camera, each pixel in an image sensor of the 3D camera must provide distance information. Accordingly, a unit for measuring a distance from a plurality of points on a surface of an object to the 3D camera is required.
Distance information about an object is generally obtained by using a binocular stereovision method using two cameras or a triangulation method using a structured light and a camera. However, according to the two methods, the accuracy of the distance information is sharply reduced when a distance between an object and a camera increases. Also, these methods are dependent on a surface state of the object, and thus precise distance information may not be obtained.
Accordingly, a time-of-flight (TOF) method has been introduced. The TOF method irradiates a laser beam on an object, and measures TOF of light until the light is received by a light receiver after being reflected off the object. According to the TOF method, a light having a certain wavelength, such as near infrared rays of 850 nm, is projected onto the object by using a light emitting diode (LED) or a laser diode (LD), the light receiver receives a light having the same wavelength and reflected from the object, and then particular processes are performed to extract distance information. Different TOF methods may be used based on the series of particular processes.
For example, an image reflected from an object may be optically modulated by using an image intensifier or another solid modulator device, and then the optically-modulated image may be captured by using an image sensor to obtain distance information based on an intensity value. Here, a super-high optical modulation speed of tens to hundreds of MHz is required to identify a phase difference or TOF of a light according to distance. Accordingly, various types of optical modulators, such as an image intensifier including a multi-channel plate (MCP), a thin modulator device using an electro-optic (EO) material, and a gallium arsenide (GaAs)-based solid modulator device, have been suggested.
The image intensifier includes a photocathode for converting a light into electrons, a MCP for amplifying the number of electrons, and a phosphor for converting the electrons back to the light. However, the image intensifier occupies a large volume, and is expensive since a high voltage of several kV is used. Also, the thin modulator device using the EO material uses a refractive index change of a nonlinear crystalline material according to a voltage as an operating principle. Such a thin modulator device using the EO material is thick and also requires a high voltage.
Recently, a GaAs semiconductor-based modulator that is easily manufactured, small, and operable with a low voltage has been suggested. The GaAs semiconductor-based modulator includes a multiple quantum well (MQW) layer between a P-electrode and an N-electrode, and uses a phenomenon of the MQW layer absorbing light when a reverse bias voltage is applied to the P- and N-electrodes. However, a bandwidth of a modulator of the GaAs semiconductor-based optical modulator is about 4 nm to about 5 nm, which is very narrow.